1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition and a buzzer and a piezoelectric actuator using the piezoelectric ceramic composition.
2. Description of the Related Art
There have been known various different piezoelectric devices formed by using a piezoelectric ceramic composition. Several such known piezoelectric devices are, for example, a piezoelectric buzzer, a piezoelectric actuator, a piezoelectric sensor and a piezoelectric ceramic filter.
In a process for attaching a piezoelectric device to a circuit board, the piezoelectric ceramic forming the piezoelectric device will be exposed to a high temperature of 200.degree. C. or higher in a reflow furnace. For this reason, it is required that the piezoelectric ceramic should have a sufficient heat resistance.
In order to increase heat resistance of a piezoelectric ceramic formed by using lead zirconate titanate (hereinafter referred to as PZT), it has been known that some effective treatments are to increase the Curie temperature of the piezoelectric ceramic and to reduce the electrical resistivity of the piezoelectric ceramic. The reason for reducing the electrical resistivity of a piezoelectric ceramic may be explained as follows. If the electrical resistivity of a piezoelectric ceramic is reduced, it may be made possible to inhibit generation of pyroelectric charges at a temperature which is lower than Curie temperature, the pyroelectric charges causing a field reversing phenomenon that will in turn cause a reversed polarization, hence resulting in a deterioration in its piezoelectric characteristics such as the piezoelectric d constant and electromechanical coupling coefficient.
Further, in order to reduce the electric resistivity without greatly reducing the Curie temperature of a PZT ceramic, there is known another method in which an extremely small amount of Cr, Mn or the like is added to the ceramic. However, if Cr, Mn or the like is added in the above ceramic, its piezoelectric characteristics will be deteriorated. In order to avoid such a deterioration, further added was Nb or Sb, or the Pb displaced with Ca, Sr, Ba, La or the like, thereby prohibiting the deterioration of the piezoelectric characteristics while at the same time keeping its electric resistivity at a relatively low value although its Curie temperature will become a little lower. As such a piezoelectric ceramic composition, there has been, for example, an improved material formed by the PZT containing La, Cr and Sb as disclosed in Japanese laid-open patent applications (Tokkai) No. 10-7460 and No. 10-95666.
However, it was found that the above discussed conventional piezoelectric ceramic composition suffers a considerable deterioration in its piezoelectric characteristics due to the presence of Cr or Mn, and it was also found that even if Nb or Sb was added or Pb was displaced by Ca, Sr, Ba, La or the like in order to inhibit deterioration in its piezoelectric characteristics, the piezoelectric characteristics were still very low, resulting in a problem that it was impossible to obtain high piezoelectric characteristics and high heat resistance, both of which are however required when forming a piezoelectric buzzer or a piezoelectric actuator.
In general, larger piezoelectric characteristics such as a piezoelectric d constant and electromechanical coupling coefficient will ensure a higher performance for a piezoelectric device such as a piezoelectric buzzer and a piezoelectric actuator. On the other hand, since the performance of a piezoelectric device is likely to be deteriorated during the attachment process carried out in a reflow furnace, and since it is very important to ensure a stable drive at a high temperature, it is required that a piezoelectric ceramic should have a high heat resistance.